1. Field of the Invention
The invention relates to an optical system of a microlithographic projection exposure system having at least one refractive optical element having stress-induced birefringence.
2. Description of Related Art
Birefringent optical materials have the property that the refractive index is anisotropic. As a result, the refractive index is dependent on the polarization of a transiting light beam and its orientation relative to the material. Because of this, a birefringent material usually splits a beam of unpolarized light into two beams having orthogonal linear polarization directions.
There may be different causes for birefringence in optical materials. Crystals belonging to the hexagonal, tetragonal or trigonal crystal systems have their atoms arranged so that light propagating in some general direction will encounter an asymmetric structure. A direction about which the atoms are arranged symmetrically is usually referred to as optic axis or as birefringence axis. In order to avoid any confusion with the optical axis denoting an axis of symmetry of a lens system, the term birefringence axis will be used hereinafter. Uniaxial crystals such as MgF2 or crystalline SiO2 have only one birefringence axis. However, also cubic crystals such as CaF2 may be birefringent, at least for a certain wavelength range. In this case the birefringence is usually referred to as intrinsic.
Apart from that, there are also non-crystalline materials which may be optically birefringent. In these cases the birefringence is caused by disturbances of the molecular order in the short-range. Such disturbances may be a result of external mechanical forces, of electrical fields or of magnetic fields. The material often loses its birefringent property when the causes of these disturbances are no longer present. If, for example, a lens mount exerts mechanical forces on a lens which produce stress-induced birefringence, this birefringence will disappear as soon as the lens mount is removed.
In the case of permanent stresses, irreversible stress-induced birefringence may be observed. This situation often occurs in blanks of quartz glass that are used for the manufacture of lenses and other refractive optical elements. The irreversible stress-induced birefringence in these blanks is usually a result of the manufacturing process. The magnitude and orientation of the birefringence then often is at least approximately rotationally symmetrical with respect to an axis of symmetry of the blank. The magnitude of the birefringence generally increases with growing distance from this axis.
In many optical systems, the stress-induced birefringence in the refractive optical elements may, although being significant, be neglected. However, if very severe demands are posed on the imaging properties of the optical system, the effects of stress-induced birefringence cannot be ignored. Examples of such optical systems are the subsystems of microlithographic projection exposure apparatuses, namely the illumination system and the projection lens. Such apparatuses are used to lithographically define the structures of highly integrated electrical circuits and other microstructured components. For this purpose, a reticle is provided containing minute structures to be imaged. These structures are illuminated by the illumination system and imaged—generally to a reduced size—by means of the projection lens on a light-sensitive layer which is deposited to a substrate, e.g. a silicon wafer. If significant birefringence occurs in the projection lens, this leads to an intolerable decrease of the contrast in the image plane of the projection lens unless suitable countermeasures are taken.
By contrast, significant birefringence in the illumination systems makes it difficult to illuminate the reticle with projection light having a defined state of polarization.
The only countermeasure against irreversible stress-induced birefringence known to date is to use blanks whose irreversible stress-induced birefringence is as small as possible. However, such blanks are expensive and often not easily available.
The problem of birefringence has also acquired a particular significance in the case of projection lenses which are designed for shorter wavelengths, e.g. 157 nm. Because of the low transparency of conventional lens materials to deep UV light, calcium fluoride (CaF2) is envisaged as a material because this crystal is still transparent even at very short UV wavelengths. However, as has now been discovered, this material is intrinsically birefringent at such short wavelengths.
In order to reduce the adverse effects of intrinsic birefringence, it has been proposed to select the crystal orientations of a plurality of optical elements made from calcium fluoride in such a way that birefringence direction distributions are obtained that are at least approximately rotationally symmetrical. In certain cases it is even possible to obtain at least a partial mutual compensation of the birefringence (or, to be more precise, of the retardances caused by the intrinsic birefringence) inherent to the individual optical elements. Arrangements of this kind are described in, for example, document WO 02/093209 A2.
However, in the general case it is not possible to achieve a complete compensation of intrinsic birefringence even if the crystal orientations are optimally selected. This is due to the fact that a complete compensation of retardances caused by intrinsic birefringence requires not only a suitable combination of the birefringence distributions, but also matching geometrical path lengths and angles of incidence of the light propagating through the crystals. To avoid any undesired residual retardances, WO 02/099500 A2 proposes an additional correcting element having a defined and carefully determined stress-induced birefringence. This stress-induced birefringence is determined such that retardances caused by intrinsic birefringence in the rest of the system are at least partially compensated for.
WO 03/046634 A1 discloses a method for compensating the birefringence caused by intrinsically birefringent crystals. One of the measures described therein is to cause a stress-induced and rotationally symmetrical birefringence in a non-crystalline material by carefully controlling the temperature during the manufacturing process.